Stars hiding in dark matter halos made early galaxies look big

Insignificant in visible light, halo stars are big in the infrared background.

When the known galaxies and stars in the right panel are subtracted, what remains is the cosmic infrared background shown at left. Astronomers have determined much of this haze is from stars in the dark matter halos of early galaxies.

The search for the earliest galaxies in the Universe is ongoing. Since these galaxies are far removed from us in time, they are very faint and very red-shifted, making it hard to determine how many there were and where they were distributed. To find these galaxies, some astronomers are looking at the Universe's infrared background across large patches of the sky. Fluctuations in the temperature of this infrared background are likely indicators of the first galaxies, which heated and ionized much of the gas in the Universe.

A new observation using data from the Spitzer infrared space telescope has found the expected signature of distant, faint galaxies. However, the magnitude of the fluctuations was surprisingly high: these early galaxies appeared bigger and brighter than expected from theory and observations at other wavelengths. In a new Nature paper, Asantha Cooray and colleagues suggest that much of this infrared radiation came from stars in the galactic halos, which were thought to be mostly dark matter.

Typical galaxies such as the Milky Way have two basic parts: the luminous portion (which is what we usually think of as the galaxy), and a dark matter halo that envelops it and contains most of the mass. Even though most of a galaxy's stars are in the luminous portion, the halo does contain a substantial number of stars, although they're at a much lower density. Recent studies have shown that halo stars contribute more to the total light profile of a galaxy than we previously thought.

The mass of the dark matter halos is thought to have been instrumental in drawing atoms into the first galaxies, a process that left its mark on the early Universe. The first stable atoms formed around 400,000 years after the Big Bang. As electrons joined with protons, they emitted light we now see as the cosmic microwave background. When the first stars and galaxies formed, however, their intense radiation stripped electrons from atoms again, an event known as reionization. According to theory, that is: while the ionized gas has been seen, the stars that drove it are distant and hard to observe.

However, the earliest stars and galaxies should contribute to the total infrared glow of the Universe, known as the cosmic near-infrared background (CNIB). ("Near-infrared" refers to wavelengths closest to visible light in the electromagnetic spectrum; in this case, the study was in the 1 to 5 micron range.) Much of the haze in the CNIB is from the Milky Way and known galaxies, but a significant portion is not associated with any obvious sources. Astronomers have postulated it must originate in either to dwarf galaxies (which are too small to be seen at significant distances) or faint galaxies from the early Universe.

Until the current study, no survey of the CNIB had sufficient resolution to distinguish between small (but relatively close) galaxies and the first to form in the Universe. The researchers decisively determined the overabundance of infrared emission was originating from an area that is too large to be dwarf galaxies. The surprise was that it was too large to be normal galaxies either—or at least the star-rich portions of those galaxies.

The current data strongly supports that assertion: if the halo stars were added to the galactic contribution, then they made up the difference between the expected infrared haze and the observed amount. The measured distribution of bright patches in the CNIB was consistent with galaxies as massive as the Andromeda Galaxy, but were generating light more than 7 billion years ago—over half the current age of the Universe.

These results provide significant additions to our understanding of early galaxies: their distribution, numbers, and the role of halo stars in their overall light profile.

Promoted Comments

Stupid question: when we're looking far away we see the universe when it was younger. But it was also smaller, and yet it's seen all around our present larger universe. Does this mean the early universe looks magnified? Do these galaxies appear larger, or more spaced out?

The "early Universe" is visible in every direction because the Universe has no center. So, the farther you look in every direction, the earlier in the Universe's history you see. Imagine the Universe we can see at any given distance as a balloon that has been inflating for billions of years, with us at the center. Sure, when the events we observe on the balloon actually happened, the balloon was smaller. But, the angular separations from our point of view at the center are no different than they were billions of years ago, so everything looks as far away from everything else as now.

Here's another way of looking at it. Take a very nearby spherical shell of objects and draw lines to the galaxies you see, from your position at the center of the shell. Now events in the early Universe are on a larger spherical shell, farther away in every direction. Continue drawing your lines out to this further shell. Would the galaxies appear more spaced out to you on this more distant shell? No, the angular separation is the same. The lines didn't change direction.

Stupid question: when we're looking far away we see the universe when it was younger. But it was also smaller, and yet it's seen all around our present larger universe. Does this mean the early universe looks magnified? Do these galaxies appear larger, or more spaced out?

Stupid question: when we're looking far away we see the universe when it was younger. But it was also smaller, and yet it's seen all around our present larger universe. Does this mean the early universe looks magnified? Do these galaxies appear larger, or more spaced out?

The "early Universe" is visible in every direction because the Universe has no center. So, the farther you look in every direction, the earlier in the Universe's history you see. Imagine the Universe we can see at any given distance as a balloon that has been inflating for billions of years, with us at the center. Sure, when the events we observe on the balloon actually happened, the balloon was smaller. But, the angular separations from our point of view at the center are no different than they were billions of years ago, so everything looks as far away from everything else as now.

Here's another way of looking at it. Take a very nearby spherical shell of objects and draw lines to the galaxies you see, from your position at the center of the shell. Now events in the early Universe are on a larger spherical shell, farther away in every direction. Continue drawing your lines out to this further shell. Would the galaxies appear more spaced out to you on this more distant shell? No, the angular separation is the same. The lines didn't change direction.

Oh I see. So in the case of a balloon expanding with me in the center, and if the galaxies were painted on the sides of the balloon, I would see the galaxies remain the same size because they would expand while being stretched. But in the case of the universe it's only the space that's expanding while the galaxies remain the same size, so they appear smaller to me as the universe expands but to me the distance between them appears the same. Right?

Oh I see. So in the case of a balloon expanding with me in the center, and if the galaxies were painted on the sides of the balloon, I would see the galaxies remain the same size because they would expand while being stretched. But in the case of the universe it's only the space that's expanding while the galaxies remain the same size, so they appear smaller to me as the universe expands but to me the distance between them appears the same. Right?

Right. They're farther away and therefore smaller (actually, mostly they appear dimmer ), but nothing appears disproportionate to you (I mean, things in the early Universe really were different than today, but after taking that into account, nothing appears disproportionate).

And it's better to think of the galaxies as being ants standing on the balloon. The ants are bound objects and so aren't stretched by the stretching of the balloon (although the balloon tries, and if it expanded fast enough, it could tear the ants apart). They're also free to move around and tend to go toward each other if they can, but it's hard because the balloon keeps moving them farther apart.

Aren't we able to see galaxies from almost the beginning of time? Like within a couple hundred million years after the Big Bang? 7 billion years seems way too young to care about. Please explain.

That's my question. I'd be surprised if galaxies only 7 billion YA were all that different from today. The big differences would be in the earlier assembly phase, say go back 10-12 billion YA and you've got galaxies that are fairly young and still assembling. Certainly the reionization of the Universe took place LONG before 7 billion YA.

I think the confusion comes in when you think of the big bang. If there was a big bang, then that sort of implies that there was at some point a singularity from which everything we can observe came from. Even assuming perfectly even distribution of parts from that big bang (unlikely), what are the odds that we're conveniently situated near the point where the big bang occurred? To me this almost sounds like astronomers from before our current heliocentric view evolved.

Once the expansion stops and we begin moving towards the big crunch in a few hundred trillion years (or however long it is from now), then we'll know.

I think the confusion comes in when you think of the big bang. If there was a big bang, then that sort of implies that there was at some point a singularity from which everything we can observe came from.

True. That singularity is now the Universe. It's not that the singularity existed somewhere and then expanded to included the Milky Way's location, or that the singularity had a center and the Milky Way is off to side of that or something.

The Big Bang had no location. It's like asking "what's the center of the surface of the Earth?" or "Why when I look in every direction do I see stuff? Is my position special?" Well, no. Everyone on Earth can see things up to their own horizon. Different locations have different "observable sections of the Earth" and no position on the Earth is favored. This isn't a perfect example, because the Earth has a rotational axis and so is not perfectly symmetric like the Universe is, but it's the general idea. It's also flawed because we think of the Earth as occupying some part of space, but in the analogy the surface of the Earth *is* space, not something that popped into existence within a larger space.

Imagine the Earth was once not here, then it was here but very small, and then it blew up to its current size and was still expanding. Where would you say the Big Bang "occurred" exactly? It really happened everywhere on the Earth's surface, simultaneously, when the whole thing popped into existence and then began expanding.

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Even assuming perfectly even distribution of parts from that big bang (unlikely), what are the odds that we're conveniently situated near the point where the big bang occurred?

That the Universe is perfectly evenly distributed (isotropic and homogenous in the lingo) is well established and explained by inflation. See the above for why the "where the Big Bang" occurred question is not really meaningful ("Why do geologists in Sydney think they're near where the Earth's surface popped into existence?" They don't; they don't think such a question is meaningful.).

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To me this almost sounds like astronomers from before our current heliocentric view evolved.

It's exactly the opposite. The mainstream view says the solar system, Milky Way, and so on occupy a tiny piece indistinguishable from the rest of the infinite or "nearly infinite" and homogenous Universe. Our place is insignificant and indistinguishable from any other.

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Once the expansion stops and we begin moving towards the big crunch in a few hundred trillion years (or however long it is from now), then we'll know.

No big crunch for us. The energy density is too low--also predicted by inflationary models.

To me this almost sounds like astronomers from before our current heliocentric view evolved.

It's exactly the opposite. The mainstream view says the solar system, Milky Way, and so on occupy a tiny piece indistinguishable from the rest of the infinite or "nearly infinite" and homogenous Universe. Our place is insignificant and indistinguishable from any other.

The nice thing about being incredibly typical is that we can look at our local neighborhood and extrapolate with some confidence. We know that local lumpiness exists (and that it's possible for a lump to evolve life on it), but overall our sky should look more or less like any other sky in the Universe.

I think the confusion comes in when you think of the big bang. If there was a big bang, then that sort of implies that there was at some point a singularity from which everything we can observe came from. Even assuming perfectly even distribution of parts from that big bang (unlikely), what are the odds that we're conveniently situated near the point where the big bang occurred? To me this almost sounds like astronomers from before our current heliocentric view evolved.

Once the expansion stops and we begin moving towards the big crunch in a few hundred trillion years (or however long it is from now), then we'll know.

I know it was answered above me but I wanted to add something:

On cosmological scales the distribution of the universe is indeed very uniform. All you have to do is look at the Hubble deep field images to see this. No matter where you point the camera there's going to be about as many galaxies occupying the space. Saying that, after the Big Bang the early universe couldn't have been perfectly uniform in distribution otherwise we wouldn't be here. Entropy says as much.

As for the big crunch, not gonna happen. We're accelerating with expansion. Vacuum energy will make sure of that.

I think the confusion comes in when you think of the big bang. If there was a big bang, then that sort of implies that there was at some point a singularity from which everything we can observe came from. Even assuming perfectly even distribution of parts from that big bang (unlikely), what are the odds that we're conveniently situated near the point where the big bang occurred? To me this almost sounds like astronomers from before our current heliocentric view evolved.

The universe does not have a center. As we understand it currently, there are, broadly speaking, two cases: If the universe is infinite in extent, then it always has been infinite, including at the Big Bang (in this scenario, it was never a point). It was infinitely dense at the Big Bang and as it has expanded since then the density has gone down. Being always infinite, defining a center doesn't make much sense. The second case, the one Einstein favored, is that the universe is finite and "closed". You can kind-of imagine this case by considering a 2D analogy: Imagine that the universe is the surface of a balloon (i.e. it contains only two spacial dimensions). In this instance the interior of the balloon has no meaning as all of the universe is the surface. If you blow air into the balloon, the universe gets bigger - i.e. more surface area. In this case the universe did start from a point. But I think that you can see that the surface of the balloon has no center and no matter what point on the surface you chose, the rest of the universe is expanding away from you in the same way. In this balloon universe, if you cold travel far enough in a "straight line" you would eventually wind up back where you started. A closed version of the real universe is an impossible to visualize 3D version of this. (By the way, if the universe is closed and small enough, then it might be possible to tell this from examining the cosmic microwave background radiation).

It is important to keep in mind that the Big Bang was not an explosion of stuff into the void - there was no void - it was the expansion of the universe itself (including space and time). Other things in the universe aren't moving away from us, space itself is getting bigger and as such it is possible for distant things to be receding from us more rapidly than the speed of light and thus forever invisible and inaccessible to us.

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Once the expansion stops and we begin moving towards the big crunch in a few hundred trillion years (or however long it is from now), then we'll know.

As best we can tell currently, this won't happen. Expansion of the universe is actually accelerating. We don't know why but it seems to be a property of space itself (out of ignorance we call it dark energy).

I read the Harvard FAQ above and this surprised me:"We know that the galaxies must extend much further than we can see, but we do not know whether the universe is infinite or not."

I thought the Hubble Ultra Deep Field image shows galaxies that are only a few hundred million years old, so there's not much further away we could look at. This FAQ says that the universe is "about 15 billion years old plus or minus a few billion", isn't that very outdated (like, 1980s outdated)?

So since we can see the earliest galaxies today, are we any closer to knowing whether the universe is infinite in extent or whether it is finite, and curves around itself? In that second case, if we look in one direction, we would see the back of our own region of the universe as it existed shortly after the Big Bang?

I think the confusion comes in when you think of the big bang. If there was a big bang, then that sort of implies that there was at some point a singularity from which everything we can observe came from. Even assuming perfectly even distribution of parts from that big bang (unlikely), what are the odds that we're conveniently situated near the point where the big bang occurred? To me this almost sounds like astronomers from before our current heliocentric view evolved.

One of the most understandable explanations (for me at least) that I've heard goes as follows:

Consider a 2D universe. Imagine that this 2D universe is elastic, like an infinite sheet of rubber. If we are a dot on this infinite sheet of rubber, in the present day we can draw a circle centered on us that extends out to ~14 billion light years that represents the *observable* universe. There's still plenty of rubber sheet in all directions that still is part of the universe, it's just that it's not currently observable.

If we now rewind time there are two things that happen: 1) The observable universe shrinks by 1 light year per year (because we can only observe light that has had time to reach us). More importantly for this discussion though, is that: 2) The rubber sheet itself shrinks. This represents the expansion of the universe.

So, if we extrapolate back to the beginning of the universe we find that the rubber sheet becomes more and more dense. From our vantage point on our little dot on the compressed rubber sheet the observable universe gets smaller and smaller (because of the two effects mentioned above), until eventually everything that we can see in the night sky today is compressed into an infinitesimal point -- a singularity. At the same time though, every other point on the infinite sheet of rubber is also compressed to a singularity.

It's a bit mind-bending to consider because we're dealing with infinities (the sheet) and infinitesimals (singularities), but dropping a dimension and thinking in terms of a rubber sheet has always made way more sense to me than other 3D analogies. Basically, the *observable* universe grew from a "part of" an infinite singularity at the Big Bang. The universe itself is and always has been infinite -- it's just the density of space itself that has changed.

What falls out of this explanation is that our current viewpoint is heliocentric by geometric necessity -- by definition we will always be at the centre of the observable universe.

This explanation is similar to the balloon analogy above, but applies regardless of the universe's topology (i.e. whether it is closed or open) in that the infinite rubber sheet may or may not be a closed surface or not. As others have said, based on observations the universe appears pretty much the same in every direction which suggests that our heliocentric viewpoint, while it is truly specific to us, is not all that unique and that we can expect the universe to look pretty much the same from any arbitrary vantage point.

Anyway, the rubber sheet comparison really crystallized how I think about the Big Bang and puts articles like this in a much more understandable context. Hope it helps!

I've never had a confusion about the center / rubber balloon thing, but the SIZE thing gets me. Wasn't the universe a lot smaller during the time this light is from? If so, wouldn't it have had time to cross the entire universe already? So what happens to it then, it just wraps around the other side and keeps on zipping through the universe again and again until the end of time? Then is this "further away" that we interpret, does it include a few round trips? Doesn't that mess with the red-shift mechanism that gives the interpretation in the first place? Or has the universe always been expanding at an appreciable fraction of the speed of light, so that the wrapping around hasn't had to happen?

I've never had a confusion about the center / rubber balloon thing, but the SIZE thing gets me. Wasn't the universe a lot smaller during the time this light is from? If so, wouldn't it have had time to cross the entire universe already? So what happens to it then, it just wraps around the other side and keeps on zipping through the universe again and again until the end of time? Then is this "further away" that we interpret, does it include a few round trips? Doesn't that mess with the red-shift mechanism that gives the interpretation in the first place? Or has the universe always been expanding at an appreciable fraction of the speed of light, so that the wrapping around hasn't had to happen?

The expansion of space is the explanation for this. For any 2 points in space there is a certain amount of space between them. ALL of space is getting bigger, so the further apart 2 things are, the more space is between them and the faster they are pushed apart. Once the distance becomes great enough this apparent velocity of recession exceeds the speed of light. You cannot ever see anything beyond that horizon.

The other thing to understand is that while the universe BEGAN at a single point the 'cosmic fireball' had finite size when it became transparent. The CMB is our view of the nearest surface of that fireball, which was impenetrable to light. The decoupling of light and matter happened everywhere at once (or nearly so) but since at any given point you are looking backwards in time as you look further out in space you always eventually see this opaque fireball, your vision of anything beyond that point is obscured (gravitational waves might allow us to observe things beyond this threshold, which is one big reason for building GW detectors).

The upshot is that we only see a portion (actually a rather small portion) of the whole universe.

Just wanted you all to know that I find this incredibly interesting and the comments here have gone a long way towards making it accessible. I've only done a bit of reading on cosmology sort of things before and simple, lucid explanations like the ones given here are really wonderful. So, you know, cheers to all of you -- and thanks.

The measured distribution of bright patches in the CNIB was consistent with galaxies as massive as the Andromeda Galaxy, but were generating light more than 7 billion years ago—over half the current age of the Universe.

MIlky Way Galaxy is 13± billion years old.

Andromeda Galaxy is 9± bilion years old.Andromeda doesn't seem to be on it's final death bed.

So given the Universe is 14± billion years old - this is basically saying that many galaxies having a rough size comparison to Andromeda formed - evolved and died out in less than 6-7 billion years.

In addition to them having been long since gone for as long.

Maybe there was more potential energy in the earlier stages of the Universe (assuming the Big Bang Theory holds). But that being said - those early galaxies would not have been consistnet with what we currently classify galaxies as. More akin to animal species evolving over time. Less time for full blown independant planetary systems to form much less stable stars.

And then we're talking of a gap of about 1 billion years for the Universe to "get it right" - relative to our galaxy forming. Unless the Universe simply threw dozens of different things onto the stove and waited to see what cooks up and how.

Their conclusions might explain one thing that they were aiming for - but it also leaves out many other accepted conclusions about our current understanding of the early Universe and throws much of them into conflict.

But this still smells of a "flat-world believer" thoery from a single vantage point.

I've never had a confusion about the center / rubber balloon thing, but the SIZE thing gets me. Wasn't the universe a lot smaller during the time this light is from? If so, wouldn't it have had time to cross the entire universe already? So what happens to it then, it just wraps around the other side and keeps on zipping through the universe again and again until the end of time? Then is this "further away" that we interpret, does it include a few round trips? Doesn't that mess with the red-shift mechanism that gives the interpretation in the first place? Or has the universe always been expanding at an appreciable fraction of the speed of light, so that the wrapping around hasn't had to happen?

The expansion of space is the explanation for this. For any 2 points in space there is a certain amount of space between them. ALL of space is getting bigger, so the further apart 2 things are, the more space is between them and the faster they are pushed apart. Once the distance becomes great enough this apparent velocity of recession exceeds the speed of light. You cannot ever see anything beyond that horizon.

The other thing to understand is that while the universe BEGAN at a single point the 'cosmic fireball' had finite size when it became transparent. The CMB is our view of the nearest surface of that fireball, which was impenetrable to light. The decoupling of light and matter happened everywhere at once (or nearly so) but since at any given point you are looking backwards in time as you look further out in space you always eventually see this opaque fireball, your vision of anything beyond that point is obscured (gravitational waves might allow us to observe things beyond this threshold, which is one big reason for building GW detectors).

The upshot is that we only see a portion (actually a rather small portion) of the whole universe.

Based on what you say - that then makes the Universe significantly larger than the accepted 13-14 billion light years (beacuse that is what we can observe) which also adds to an existense of the Universe prior to the Big Bang - as something had to spark it in the first place. And for all we know - what we call the Big Bang only happened in a single area of the total Universe and since it was so massive and obscures anything beyond the CMB that we can observe from Earth - we simply can't see it. Not too different than an explosion with a huge fireball and lots of smoke - not being able to see past the smoke. Something obviously exists on the other side of the smoke - we just can't see it.

I've never had a confusion about the center / rubber balloon thing, but the SIZE thing gets me. Wasn't the universe a lot smaller during the time this light is from? If so, wouldn't it have had time to cross the entire universe already? So what happens to it then, it just wraps around the other side and keeps on zipping through the universe again and again until the end of time? Then is this "further away" that we interpret, does it include a few round trips? Doesn't that mess with the red-shift mechanism that gives the interpretation in the first place? Or has the universe always been expanding at an appreciable fraction of the speed of light, so that the wrapping around hasn't had to happen?

The expansion of space is the explanation for this. For any 2 points in space there is a certain amount of space between them. ALL of space is getting bigger, so the further apart 2 things are, the more space is between them and the faster they are pushed apart. Once the distance becomes great enough this apparent velocity of recession exceeds the speed of light. You cannot ever see anything beyond that horizon.

The other thing to understand is that while the universe BEGAN at a single point the 'cosmic fireball' had finite size when it became transparent. The CMB is our view of the nearest surface of that fireball, which was impenetrable to light. The decoupling of light and matter happened everywhere at once (or nearly so) but since at any given point you are looking backwards in time as you look further out in space you always eventually see this opaque fireball, your vision of anything beyond that point is obscured (gravitational waves might allow us to observe things beyond this threshold, which is one big reason for building GW detectors).

The upshot is that we only see a portion (actually a rather small portion) of the whole universe.

Based on what you say - that then makes the Universe significantly larger than the accepted 13-14 billion light years (beacuse that is what we can observe) which also adds to an existense of the Universe prior to the Big Bang - as something had to spark it in the first place. And for all we know - what we call the Big Bang only happened in a single area of the total Universe and since it was so massive and obscures anything beyond the CMB that we can observe from Earth - we simply can't see it. Not too different than an explosion with a huge fireball and lots of smoke - not being able to see past the smoke. Something obviously exists on the other side of the smoke - we just can't see it.

We're inside the Delphic expanse! (I loved that season of ST:Enterprise)

While i agree that thinking about an expanding universe makes more sense, would it be wrong to say that the universe is staying the same size and that are the objects in it that are getting smaller and lighter? We cannot distinguish the two alternatives anyway, can we?

Why is it that the universe is expanding? If you start with all the mass of the universe in a single point wouldn't the gravitational pull be infinite (ala a super super.... super massive black hole).

Not all of it was in a single point but I guess that most likely reason was inflation period where universe expanded too fast for the gravity to really matter. What set it off nobody knows.

As to analogies I really like the bread with raisins in it. As it grows all of the bread expands including the space between the raisins (galaxies) who are held together by forces stronger than the expansion.

I think the biggest problem with understanding the notion of the expanding universe and the Big Bang is that the universe is not Euclidean, but most people's understanding of geometry (whether schooled or intuitive) is based entirely on Euclidean geometry, and the analogies scientists use to try to explain the universe are also based in a Euclidean universe -- the surface of a balloon is non-euclidean, but that balloon is still embedded in a larger, Euclidean space, inviting people to thinking of the Big Bang in a similar way.

fferitt25 wrote:

Based on what you say - that then makes the Universe significantly larger than the accepted 13-14 billion light years (beacuse that is what we can observe) which also adds to an existense of the Universe prior to the Big Bang - as something had to spark it in the first place. And for all we know - what we call the Big Bang only happened in a single area of the total Universe and since it was so massive and obscures anything beyond the CMB that we can observe from Earth - we simply can't see it. Not too different than an explosion with a huge fireball and lots of smoke - not being able to see past the smoke. Something obviously exists on the other side of the smoke - we just can't see it.

The Big Bang does not imply that the universe that we can see (the "observable universe") is or must be the entire universe. It does not imply that if there is a universe past the 14 billion ly distance we can see it must be older than 14 billion years.

The Big Bang did not happen in the universe. It happened to the universe. It's not an explosion in space, it's an "explosion" of space.

The problem comes when you try to picture this in a Euclidean universe. How can the universe (or just the bubble constrained by our local Big Bang in your formulation) "expand" without anything to expand into? If you have something that is 2 meters across, and you expand it so that it is 3 meters across, then it necessarily pushes 1 meter further into whatever was adjacent to your original 2-meter-wide something.

This is Euclidean thinking. Understandable and natural -- maybe even instinctual -- but not how the universe actually works. Einstein gives an elegant proof for how this must be the case (given the postulates of General Relativity) in his book "Relativity: The Special and the General Theory".

In a non-Euclidean geometry, this is all much easier to understand. It's not "space" that's "expanding" into... something. It's the metric -- the thing which is used to define locations in space time, and thus the distance between points -- that is changing. You don't need to make the universe get "bigger" and occupy more of some meta-universe. You just need to alter the metric such that two points, despite not actually 'moving', are now 3 meters apart when before they were 2.

That breaks Euclidean geometry. But in Gaussian geometry it's perfectly fine and understandable.

Too bad non-Euclidean geometry is hard to understand. So I'm not sure if I helped. =D

I've never had a confusion about the center / rubber balloon thing, but the SIZE thing gets me. Wasn't the universe a lot smaller during the time this light is from? If so, wouldn't it have had time to cross the entire universe already? So what happens to it then, it just wraps around the other side and keeps on zipping through the universe again and again until the end of time? Then is this "further away" that we interpret, does it include a few round trips? Doesn't that mess with the red-shift mechanism that gives the interpretation in the first place? Or has the universe always been expanding at an appreciable fraction of the speed of light, so that the wrapping around hasn't had to happen?

The expansion of space is the explanation for this. For any 2 points in space there is a certain amount of space between them. ALL of space is getting bigger, so the further apart 2 things are, the more space is between them and the faster they are pushed apart. Once the distance becomes great enough this apparent velocity of recession exceeds the speed of light. You cannot ever see anything beyond that horizon.

The other thing to understand is that while the universe BEGAN at a single point the 'cosmic fireball' had finite size when it became transparent. The CMB is our view of the nearest surface of that fireball, which was impenetrable to light. The decoupling of light and matter happened everywhere at once (or nearly so) but since at any given point you are looking backwards in time as you look further out in space you always eventually see this opaque fireball, your vision of anything beyond that point is obscured (gravitational waves might allow us to observe things beyond this threshold, which is one big reason for building GW detectors).

The upshot is that we only see a portion (actually a rather small portion) of the whole universe.

Based on what you say - that then makes the Universe significantly larger than the accepted 13-14 billion light years (beacuse that is what we can observe) which also adds to an existense of the Universe prior to the Big Bang - as something had to spark it in the first place. And for all we know - what we call the Big Bang only happened in a single area of the total Universe and since it was so massive and obscures anything beyond the CMB that we can observe from Earth - we simply can't see it. Not too different than an explosion with a huge fireball and lots of smoke - not being able to see past the smoke. Something obviously exists on the other side of the smoke - we just can't see it.

The actual size of the universe is much more than 13-14 billion light years, yes. The concept of 'distance' at the largest scales is a bit slippery, but current estimates put the size of the universe at a minimum of 90 billion light years. CMB photons have been traveling for 13.7 billion years, but the locations that they left from are FAR further away NOW than that.

None of this says anything about anything prior to the Big Bang. It is indeed possible that the area we see is only PART of a larger universe, yes. Our part was subject to inflation, which may be propagating endlessly onward across the larger total universe for instance. Other inflations could have happened in other places, but those locations are permanently cut off from us in space and time.

Indeed, given that there are parts of the universe we can't we can't know what is there. It has been suggested that potentially over VERY large scales the laws of physics could vary. We can't SEE any area where they deviate, but if you could go look at the universe from 80 or 100 or 1000 billion light years from here the rules could be different.

As to what might have touched it off, well we can only speculate about the laws which govern whatever the universe arose from. Ultimately this becomes an epistemological discussion, what are the fundamental rules and what/where is the 'fundamental underlying' universe. It is no less intellectually compelling to say that our universe arose from NOTHING than it is to say it arose from some more fundamental reality. You could also simply posit that universes beget more universes ad infinitum, but again this is no more intellectually compelling.

I think the biggest problem with understanding the notion of the expanding universe and the Big Bang is that the universe is not Euclidean,

While I get your general point, to the best of my knowledge space is flat, which is essentially the same thing as being Euclidean, is it not?

Well, except we're not talking about 'space', we're talking about spacetime, which is definitely not Euclidean. It is of course 4 dimensional for one thing, but beyond that 3 of the dimensions are space-like and one is time-like. Furthermore each independent reference frame has its own set of coordinates which are related by a transform (differences in velocity are actually just differences between the relative orientations of the different reference frames).

I think the biggest problem with understanding the notion of the expanding universe and the Big Bang is that the universe is not Euclidean,

While I get your general point, to the best of my knowledge space is flat, which is essentially the same thing as being Euclidean, is it not?

One of the hardest things to convey in all of the analogies about space is that space is curved and that time is not moving at a constant rate. Mass deforms the surrounding space, sort of like putting a bowling ball on a trampoline. Roll another ball in a straight line across the trampoline and it will curve in toward the bowling ball (actually both balls move, the amount depending on their relative masses). This is not a perfect analogy, because it depends on your intuitive knowledge that things trend "down" which depends implicitly on the Earth's gravity, but I hope it was illistrative.

All bodies known seem to have some spin. From galaxies to electrons this seems to hold true. Does the universe itself have a spin and if so (or not) how could we tell?

Well, this is a significant question actually. Some claims have been made based on galaxy rotation surveys (Longo), but the question is far from answered. The main thing that would result from a universal rotation would be that the universe is anisotropic (some directions are different from others). Of course it may be impossible to determine if the WHOLE universe is rotating or only that part we can see.

All bodies known seem to have some spin. From galaxies to electrons this seems to hold true. Does the universe itself have a spin and if so (or not) how could we tell?

Well, this is a significant question actually. Some claims have been made based on galaxy rotation surveys (Longo), but the question is far from answered. The main thing that would result from a universal rotation would be that the universe is anisotropic (some directions are different from others). Of course it may be impossible to determine if the WHOLE universe is rotating or only that part we can see.

If whole-universe rotation could be shown - which I'm not sure that it could be[1] - it would also put an end to debate over Mach's principle (proving Einstein wrong).

[1] I'm not saying that the whole universe couldn't be rotating, just that I'm not sure that it could be shown to be. If you know how... please do it! This is a really big question. Showing that a portion of the universe (including the whole visible universe) has net angular momentum is clearly possible.

All bodies known seem to have some spin. From galaxies to electrons this seems to hold true. Does the universe itself have a spin and if so (or not) how could we tell?

Well, this is a significant question actually. Some claims have been made based on galaxy rotation surveys (Longo), but the question is far from answered. The main thing that would result from a universal rotation would be that the universe is anisotropic (some directions are different from others). Of course it may be impossible to determine if the WHOLE universe is rotating or only that part we can see.

If whole-universe rotation could be shown - which I'm not sure that it could be - it would also put an end to debate over Mach's principle (proving Einstein wrong).

Rotation relative to what? Therin lies the rub- if the universe really is all there is, then it can't really rotate. Stuff inside it can, though.

All bodies known seem to have some spin. From galaxies to electrons this seems to hold true. Does the universe itself have a spin and if so (or not) how could we tell?

Well, this is a significant question actually. Some claims have been made based on galaxy rotation surveys (Longo), but the question is far from answered. The main thing that would result from a universal rotation would be that the universe is anisotropic (some directions are different from others). Of course it may be impossible to determine if the WHOLE universe is rotating or only that part we can see.

If whole-universe rotation could be shown - which I'm not sure that it could be[1] - it would also put an end to debate over Mach's principle (proving Einstein wrong).

[1] I'm not saying that the whole universe couldn't be rotating, just that I'm not sure that it could be shown to be. If you know how... please do it! This is a really big question. Showing that a portion of the universe (including the whole visible universe) has net angular momentum is clearly possible.

edit: added footnote

Indeed there is the question of Mach's Principle. People are divided on that. Some people believe that the Principle would be violated, others point out that in a sense the rotation is relative to what (in a bit more technical construction). The Godel Dust model is an interesting starting point there. Other similar rotating universe models exist though. Some have rather unusual properties.

Constantly amazed, am I, at the new revelations daily. Follow cosmology with a rabid appetite and after several years...Could it be: Humanity exists and serves only to prove that the Universe is becoming self-aware. To mangle a famous philosopher, perhaps we see, therefore we are. So the more we perceive the more is created?Could it be: That we are missing an entire generation (or more) of "created Universe" in which stars condensed and henceforth "evaporated" leaving only this 'dark matter' as evidence of their passing?Are we sure that the observed universe is the only one that ever existed here? Or, like fossil dinosaurs, are there things in evidence that point to an earlier generation of stars that occupied our heavens before even this time and place we observe?

I keep reading here that "we can only observe a small part of the universe". But if we can see up to the opaque part that existed just after the Big Bang, are we not seeing almost all of it? Granted, it's larger NOW than what we can see, we are seeing ALL of it, albeit an outdated view of the oldest parts.

I keep reading here that "we can only observe a small part of the universe". But if we can see up to the opaque part that existed just after the Big Bang, are we not seeing almost all of it? Granted, it's larger NOW than what we can see, we are seeing ALL of it, albeit an outdated view of the oldest parts.

No, because our line of sight leads back to a point where we were EMBEDDED WITHIN an opaque fireball that filled all of space. In fact all you can see is just a very small local region of that fireball. Furthermore most of the space that existed at that time is receding from us so fast that we will never be able to see it.